Transversely-excited film bulk acoustic resonator with half-lambda dielectric layer

ABSTRACT

Acoustic resonator devices and filters are disclosed. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. The piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode in the diaphragm. A half-lambda dielectric layer is formed on one of the front surface and back surface of the piezoelectric plate.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application 62/818,571, filed Mar. 14, 2019, entitled XBAR WITH HALF-LAMBDA OVERLAYER, which is incorporated herein by reference. This patent is related to application Ser. No. 16/230,443, filed Dec. 21, 2018, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, now U.S. Pat. No. 10,491,192.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic wave resonators, and specifically to filters for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a pass-band or stop-band depend on the specific application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application.

RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements.

Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. The current LTE™ (Long Term Evolution) specification defines frequency bands from 3.3 GHz to 5.9 GHz. Some of these bands are not presently used. Future proposals for wireless communications include millimeter wave communication bands with frequencies up to 28 GHz.

High performance RF filters for present communication systems commonly incorporate acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave BAW) resonators, film bulk acoustic wave resonators (FBAR), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectional views of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of the XBAR of FIG. 1.

FIG. 3 is an expanded schematic cross-sectional view of a portion of an improved XBAR including a “half-lambda” dielectric layer.

FIG. 4 is a chart comparing the admittances of an XBAR with a half-lambda dielectric layer and a conventional XBAR.

FIG. 5 is a cross-sectional view of an XBAR with a half-lambda dielectric layer with contours representing stress at the resonance frequency.

FIG. 6 is a chart comparing the admittances of three XBARs with half-lambda AlN layers.

FIG. 7 is a chart comparing the admittances of three XBARs with half-lambda SiO₂ layers.

FIG. 8 is a chart comparing the admittances of three other XBARs with half-lambda SiO₂ layers.

FIG. 9 is a chart of the admittance of an XBARs with an overly thin half-lambda SiO₂ layers.

FIG. 10 is a chart of the admittance of an XBARs with an overly thick half-lambda SiO₂ layers.

FIG. 11 is a chart of the temperature coefficient of frequency of an XBAR as a function of SiO₂ thickness.

FIG. 12 is a schematic circuit diagram and layout of a filter using XBARs.

FIG. 13 is a flow chart of a process for fabricating an XBAR including a half-lambda dielectric layer.

Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digit is the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonal cross-sectional views of a transversely-excited film bulk acoustic resonator (XBAR) 100 as described in U.S. Pat. No. 10,491,192. XBAR resonators such as the resonator 100 may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suited for use in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on a surface of a piezoelectric plate 110 having parallel front and back surfaces 112, 114, respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. In the examples presented in this patent, the piezoelectric plates are Z-cut, which is to say the Z axis is normal to the front and back surfaces 112, 114. However, XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to a surface of the substrate 120 except for a portion of the piezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140 formed in the substrate. The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm” 115 due to its physical resemblance to the diaphragm of a microphone. As shown in FIG. 1, the diaphragm 115 is contiguous with the rest of the piezoelectric plate 110 around all of a perimeter 145 of the cavity 140. In this context, “contiguous” means “continuously connected without any intervening item”.

The substrate 120 provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back surface 114 of the piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 may be grown on the substrate 120 or attached to the substrate in some other manner. The piezoelectric plate 110 may be attached directly to the substrate or may be attached to the substrate 120 via one or more intermediate material layers.

“Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity 140 may be a hole completely through the substrate 120 (as shown in Section A-A and Section B-B) or a recess in the substrate 120. The cavity 140 may be formed, for example, by selective etching of the substrate 120 before or after the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigital transducer (IDT) 130. The IDT 130 includes a first plurality of parallel fingers, such as finger 136, extending from a first busbar 132 and a second plurality of fingers extending from a second busbar 134. The first and second pluralities of parallel fingers are interleaved. The interleaved fingers overlap for a distance AP, commonly referred to as the “aperture” of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites a primary acoustic mode within the piezoelectric plate 110. The primary acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate 110, which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on the portion 115 of the piezoelectric plate that spans, or is suspended over, the cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular shape with an extent greater than the aperture AP and length L of the IDT 130. A cavity of an XBAR may have a different shape, such as a regular or irregular polygon. The cavity of an XBAR may more or fewer than four sides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of the IDT fingers is greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 110. An XBAR may have hundreds, possibly thousands, of parallel fingers in the IDT 110. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of an XBAR 200 which may be the XBAR 100 of FIG. 1. The piezoelectric plate 210 is a single-crystal layer of piezoelectrical material having a front surface 214 and a back surface 216. The thickness is between the front surface 214 and the back surface 216 may be, for example, 100 nm to 1500 nm. When used in filters for 5G NR (fifth generation new radio) and Wi-Fi™ bands from 3.3 GHZ to 6 GHz, the thickness ts may be, for example, 280 nm to 550 nm.

The IDT fingers 238 may be aluminum, a substantially aluminum alloy, copper, a substantially copper alloy, tungsten, molybdenum, beryllium, gold, or some other conductive material. Thin (relative to the total thickness of the conductors) layers of other metals, such as chromium or titanium, may be formed under and/or over the fingers to improve adhesion between the fingers and the piezoelectric plate 210 and/or to passivate or encapsulate the fingers. The busbars (132, 134 in FIG. 1) of the IDT may be made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. Dimension w is the width or “mark” of the IDT fingers. The IDT of an XBAR differs substantially from the IDTs used in surface acoustic wave (SAW) resonators. In a SAW resonator, the pitch of the IDT is one-half of the acoustic wavelength at the resonance frequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e. the mark or finger width is about one-fourth of the acoustic wavelength at resonance). In an XBAR, the pitch p of the IDT is typically 2 to 20 times the width w of the fingers. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness ts of the piezoelectric plate 210. The width of the IDT fingers in an XBAR is not constrained to one-fourth of the acoustic wavelength at resonance. For example, the width of XBAR IDT fingers may be 500 nm or greater, such that the IDT can be fabricated using optical lithography. The thickness tm of the IDT fingers may be from 100 nm to about equal to the width w. The thickness of the busbars (132, 134 in FIG. 1) of the IDT may be the same as, or greater than, the thickness tm of the IDT fingers.

A shear bulk acoustic wave (BAW) propagating normal to the surfaces of a piezoelectric plate will reflect from the surfaces and resonate, or form a standing wave, when the thickness ts of the piezoelectric plate is an integer multiple of one-half the wavelength X of the acoustic wave. The longest wavelength/lowest frequency where such a resonance occurs is the shear BAW fundamental resonance at a frequency f₀ and a wavelength λ_(0,s) equal to twice the thickness ts of the piezoelectric plate. The terminology “λ_(0,s)” means the wavelength, in the piezoelectric plate, of the shear BAW fundamental (0 order) resonance of the piezoelectric plate. The wavelength of the same acoustic wave (i.e. a shear BAW propagating in the same direction with the same frequency) may be different in other materials. The frequency f₀ may be determined by dividing the velocity of the shear BAW in the piezoelectric plate by the wavelength λ_(0,s). The shear BAW fundamental resonance of the piezoelectric plate is not the same as the resonance of the XBAR device 200, which is influenced by the IDT structure.

FIG. 3 shows a detailed schematic cross-sectional view of an XBAR that incorporates a “half-lambda” dielectric layer. FIG. 3 specifically shows an XBAR 300 with a thick dielectric layer 350 on the front side (i.e. the side facing away from the substrate; the upper side as shown in FIG. 3) of a piezoelectric plate 310. A comparable dielectric layer on the back side 316 of the piezoelectric plate 310 could be used instead of the dielectric layer 350. On a larger scale, the XBAR 300 with the thick dielectric layer 350 is similar to the XBAR 100 of FIG. 1. FIG. 3 also shows two IDT fingers 338 as previously described. Dimension p is the center-to-center spacing or “pitch” of the IDT fingers, and dimension w is the width or “mark” of the IDT fingers.

The piezoelectric plate 310 is a thin single-crystal layer of a piezoelectric material such as lithium niobate or lithium tantalate. The piezoelectric plate 310 is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces 314, 316 is known and consistent. The thickness ts of the piezoelectric plate 310 may be, for example, 100 nm to 1500 nm.

The dielectric layer 350 may be nearly any dielectric material such as SiO₂, Si₃N₄, Al₂O₃, AlN. and other dielectric materials. As will be discussed, particular benefits may accrue when the dielectric material is or contains AlN and when the dielectric material is SiO₂.

The thickness ts of the piezoelectric plate 310 and the thickness td of the dielectric layer 350 are configured such a shear BAW propagating normal to the surfaces 316 and 352 forms a full-cycle standing wave between the surfaces 316 and 352 at a predetermined frequency, which may be slightly less than the desired resonance frequency of the XBAR device 300. In other words, the shear BAW second overtone resonance occurs at the predetermined frequency. By definition, the thickness ts of the piezoelectric plate is one-half of λ_(0,s), which, as previously described, is the wavelength of the shear BAW fundamental resonance of the piezoelectric plate 310 in the absence of the dielectric layer 350. Nominally the thickness td of the dielectric layer 350 is one-half of λ_(0,d), where λ_(0,d) is the wavelength of the same bulk BAW in the dielectric layer 350. In this case, each of the piezoelectric plate 310 and the dielectric layer 350 will contain a half cycle standing wave at the frequency f₀, which is now the frequency of the second overtone resonance. λ_(0,d) is equal to λ_(0,s), times the ratio of the velocity of the shear acoustic wave in the dielectric layer 350 to the velocity of the shear acoustic wave in the piezoelectric plate 310. For a relatively slow dielectric material, such as SiO₂, λ_(0,d) may be equal to or slightly greater than λ_(0,s). In this case, the thickness td of the dielectric layer 350 may be equal to or slightly greater than ts. For a relatively fast dielectric material, such as Si₃N₄ or AlN, λ_(0,d) may be substantially greater than λ_(0,s). In this case, the thickness td of the dielectric layer 350 will be proportionally greater than ts.

While the dielectric layer 350 is referred to herein as a “half-lambda” dielectric layer, the thickness td of the dielectric layer need not be exactly λ_(0,d)/2. The thickness td may differ from λ_(0,d)/2 so long as the combined thicknesses of the piezoelectric plate 310 and the dielectric layer 350 are such that the second overtone resonance of the bulk shear wave occurs at the predetermined frequency. Simulation results, some of which will be discussed subsequently, show that dielectric layer thickness with a range defined by

0.85λ_(0,d)≤2td≤1.15λ_(0,d)  (1)

results in XBARs with low spurious modes and consistent electromechanical coupling. Values of td outside of this range result in reduced electromechanical coupling and increased spurious modes. Varying td within this range allows tuning the resonant frequency of an XBAR by about 10%, which is sufficient to establish the necessary frequency offset between shunt and series resonators for many filter applications.

In FIG. 3, the dielectric layer 350 is shown deposited over and between the IDT fingers 338. In other embodiments, a half-lambda dielectric layer may be formed only between the IDT fingers. The half-lambda dielectric layer 350 may be a single layer or two or more layers of different dielectric materials having similar acoustic impedances.

A primary benefit of incorporating the half-lambda dielectric layer 350 into the XBAR 300 is the increased thickness of the diaphragm. Depending on the materials used in the half-lambda dielectric layer 350, the thickness of the diaphragm of the XBAR 300 may be two to three times the thickness of the diaphragm 115 of the XBAR 100 of FIG. 1. A thicker diaphragm is stiffer and less likely to bow or distort with changes in temperature.

The thicker diaphragm of the XBAR 300 will also have higher thermal conductivity, particularly if the half-lambda dielectric layer 350 is or includes a high thermal conductivity dielectric material such as aluminum nitride. Higher thermal conductivity results in more efficient removal of heat from the diaphragm, which may allow the use of a smaller resonator area for a given heat load or power dissipation.

The XBAR 300 will also have higher capacitance per unit area compared with the XBAR 100 of FIG. 1 (for the same IDT pitch). Resonator capacitance is a circuit design issue. In particular, RF filters using acoustic resonators are typically subject to a requirement that the input and output impedances of the filter match a defined value (commonly 50 ohms). This requirement dictates minimum capacitance values for some or all of the resonators in a filter. The higher capacitance per unit area of the XBAR 300 with partial Bragg reflectors allows the use of a smaller resonator area for any required capacitance value.

An XBAR with a half-lambda dielectric layer on the back side of the piezoelectric plate 310 (not shown) will have improved stiffness and thermal conductivity, but only slightly increased capacitance per unit area.

FIG. 4 is a chart 400 comparing the admittance of an XBAR with a half-lambda dielectric layer and a conventional XBAR. The data shown in FIG. 4 and following FIG. 6 and FIG. 7 are results of simulation of the XBAR devices using a finite element method. The solid line 410 is a plot of the magnitude of admittance as a function of frequency for an XBAR including a half-lambda dielectric layer. The piezoelectric plate is lithium niobate 400 nm thick. The IDT is aluminum 100 nm thick. The pitch and mark of the IDT fingers are 4.25 μm and 1.275 μm, respectively. The half-lambda dielectric layer consists of a layer of Si₃N₄ 350 nm thick and a layer of AlN 350 nm thick. The resonance frequency is 4.607 GHz and the anti-resonance frequency is 4.862 GHz. The different between the anti-resonance and resonance frequencies is 255 MHz, or about 5.4% of the average of the resonance and anti-resonance frequencies.

The dashed line 420 is a plot of the magnitude of admittance as a function of frequency for a conventional XBAR. The piezoelectric plate is lithium niobate 400 nm thick. The IDT is aluminum 100 nm thick. The pitch and mark of the IDT fingers are 3.7 μm and 0.47 μm, respectively. The resonance frequency is 4.71 GHz and the anti-resonance frequency is 5.32 GHz. The different between the anti-resonance and resonance frequencies is 610 MHz, or about 12.2% of the average of the resonance and anti-resonance frequencies. The admittance of the conventional XBAR (dashed line 420) exhibits some spurious modes between the resonance and anti-resonance frequencies that are not present in the admittance of the XBAR with the half-lambda dielectric layer (solid line 410).

The incorporation of a half-lambda dielectric layer in the XBAR device 300 results in a stiffer diaphragm with higher thermal conductivity and potentially lower excitation of spurious modes compared to a conventional XBAR device. These benefits come at the cost of reducing electromechanical coupling and correspondingly lower difference between the resonance and anti-resonance frequencies.

FIG. 5 a cross-sectional view of an XBAR 500 with a half-lambda dielectric layer with contours representing the stress at the resonance frequency. The piezoelectric plate 510 is lithium niobate 400 nm thick. The IDT fingers 538 are aluminum 100 nm thick. The half-lambda dielectric layer 550 consists of a layer 552 of Si₃N₄ 350 nm thick and a layer 554 of AlN 350 nm thick.

The stress in the XBAR 500 at the resonance frequency is illustrative of a full-cycle standing wave between the surfaces of the device. The stress is highest near the center of the thickness of the piezoelectric plate 510 and near the center of the dielectric layer 550, corresponding to the peaks of the two half-cycles of the standing wave. The stress is lowest at the surfaces of the device and near the boundary between the piezoelectric plate 510 and near the center of the dielectric layer 550, corresponding to the zero crossings of the standing wave.

FIG. 6 is a chart 600 illustrating the use of pitch and dielectric layer thickness to tune the resonance and anti-resonance frequencies of an XBAR with a half-lambda dielectric layer. The solid line 610 is a plot of the magnitude of admittance as a function of frequency for an XBAR with pitch and mark of 4.25 μm and 1.275 μm, respectively. The piezoelectric plate is lithium niobate 400 nm thick. The IDT is aluminum 100 nm thick. The half-lambda dielectric layer consists of a layer of Si₃N₄ 700 nm thick. The resonance frequency is 4.513 GHz and the anti-resonance frequency is 4.749 GHz. The difference between the anti-resonance and resonance frequencies is 236 MHz, or about 5.1% of the average of the resonance and anti-resonance frequencies.

The dotted line 620 is a plot of the magnitude of admittance as a function of frequency for a similar XBAR with the same construction except the pitch and mark of the IDT fingers are 3.75 μm and 1.31 μm, respectively. The resonance frequency is 4.557 GHz and the anti-resonance frequency is 4.795 GHz. Changing the IDT pitch from 4.25 μm to 3.75 μm increases the resonance and anti-resonance frequencies by about 45 MHz. Varying the pitch over a range from 3 μm to 5 μm will provide a tuning range of about 200 MHz.

The dashed line 630 is a plot of the magnitude of admittance as a function of frequency for a similar XBAR. The pitch and mark of the IDT fingers are 4.25 μm and 1.275 μm, respectively, and the dielectric layer includes a layer of Si₃N₄ 700 nm thick plus a 50 nm layer of SiO₂. The resonance frequency is 4.400 GHz and the anti-resonance frequency is 4.626 GHz. Adding the 50 nm “tuning layer” reduces the resonance and anti-resonance frequencies by about 110 MHz.

FIG. 7 is another chart 700 illustrating the use of pitch and dielectric layer thickness to tune the resonance and anti-resonance frequencies of an XBAR with a half-lambda dielectric layer. The solid line 710 is a plot of the magnitude of admittance as a function of frequency for an XBAR with pitch and mark of 4.25 μm and 1.275 μm, respectively. The piezoelectric plate is lithium niobate 400 nm thick. The IDT is aluminum 100 nm thick. The half-lambda dielectric layer consists of a layer of SiO₂ 400 nm thick. The resonance frequency is 4.705 GHz and the anti-resonance frequency is 5.108 GHz. The difference between the anti-resonance and resonance frequencies is 403 MHz, or about 8.2% of the average of the resonance and anti-resonance frequencies.

The dotted line 720 is a plot of the magnitude of admittance as a function of frequency for a similar XBAR with the same construction except the pitch and mark of the IDT fingers are 3.75 μm and 1.31 μm, respectively. The resonance frequency is 4.740 GHz and the anti-resonance frequency is 5.137 GHz. Changing the IDT pitch from 4.25 μm to 3.75 μm increases the resonance and anti-resonance frequencies by about 35 MHz. Varying the pitch over a range from 3 μm to 5 μm will provide a tuning range of about 100 MHz.

The dashed line 730 is a plot of the magnitude of admittance as a function of frequency for a similar XBAR with the same construction except the pitch and mark of the IDT fingers are 4.25 μm and 1.275 μm, respectively, and the dielectric layer is of SiO₂ 450 nm thick. The resonance frequency is 4.512 GHz and the anti-resonance frequency is 4.905 GHz. The difference between the anti-resonance and resonance frequencies is 393 MHz, or about 8.3% of the average of the resonance and anti-resonance frequencies. Increasing the thickness of the dielectric layer by 50 nm reduces the resonance and anti-resonance frequencies by about 190 MHz without reducing electromechanical coupling.

FIG. 8 is another chart 800 illustrating the use of dielectric layer thickness to tune the resonance and anti-resonance frequencies of an XBAR with a half-lambda dielectric layer. The dotted line 810 is a plot of the magnitude of admittance as a function of frequency for an XBAR with pitch and mark of 4.25 μm and 1.275 μm, respectively. The piezoelectric plate is lithium niobate 400 nm thick. The IDT is aluminum 100 nm thick. The half-lambda dielectric layer consists of a layer of SiO₂ 425 nm thick. This example represents the case where the thickness of the dielectric layer td equals λ_(0,d)/2.

The solid line 820 is a plot of the magnitude of admittance as a function of frequency for a similar XBAR with the same construction except the dielectric layer is SiO₂ 375 nm thick. In this case td=0.88(λ_(0,d)/2). The dashed line 830 is a plot of the magnitude of admittance as a function of frequency for a similar XBAR with the same construction except the dielectric layer is SiO₂ 475 nm thick. In this case td=1.12(λ_(0,d)/2). Varying the SiO₂ from 375 nm to 475 nm shifts the resonance and anti-resonance frequencies by about 400 MHz while maintaining electromechanical coupled and without introducing objectionable spurious modes.

Assuming a 400 nm thick lithium niobate piezoelectric plate, the range for td expressed in equation (1) corresponds to about 350 nm to 500 nm. This range may be expressed in terms of the thickness ts of the piezoelectric plate as follows:

0.875ts≤td≤1.25ts.  (2)

It is expected that this range will apply to any thickness for the lithium niobate piezoelectric plate.

FIG. 9 is a chart 900 illustrating the effect of an overly thin “half-lambda” dielectric layer. The solid line 910 is a plot of the magnitude of admittance as a function of frequency for an XBAR having the same construction as the devices of FIG. 8, with the SiO2 dielectric layer thickness reduced to 325 nm. In this case, td=0.76(λ_(0,d)/2). Reducing the dielectric layer thickness to this extent results in reduced electromechanical coupling and very large spurious modes below the resonance frequency of the device.

FIG. 10 is a chart 1000 illustrating the effect of an overly thick “half-lambda” dielectric layer. The solid line 910 is a plot of the magnitude of admittance as a function of frequency for an XBAR having the same construction as the devices of FIG. 8, with the SiO2 dielectric layer thickness increased to 525 nm. In this case, td=1.24(λ_(0,d)/2). Increasing the dielectric layer thickness to this extent results in reduced electromechanical coupling and very large spurious modes above the resonance frequency of the device.

The temperature coefficient of frequency of SiO2 and the temperature coefficient of frequency of lithium niobate have similar magnitude and opposing signs. XBAR devices with an SiO2 half-lambda dielectric layer will have substantially less frequency variation with temperature than conventional XBAR devices.

FIG. 11 is a chart of the temperature coefficient of frequency of an XBAR as a function of SiO₂ thickness. Specifically, the solid line 1110 is a plot of temperature coefficient of the anti-resonance frequency for the XBAR devices whose admittance characteristics were previously shown in FIG. 7 and FIG. 8. The dashed line 1120 is a plot of temperature coefficient of the resonance frequency of the same devices. Simulation results show that a conventional XBAR device without a dielectric layer has a temperature coefficient of frequency around −113 ppm/C°. The presence of the SiO2 half-lambda dielectric layer reduces the magnitude of the temperature coefficient of frequency by a factor of about 3.

FIG. 12 is a schematic circuit diagram of a band-pass filter 1200 using five XBARs X1-X5. The filter 1200 may be, for example, a band n79 band-pass filter for use in a communication device. The filter 1200 has a conventional ladder filter architecture including three series resonators X1, X3, X5 and two shunt resonators X2, X4. The three series resonators X1, X3, X5 are connected in series between a first port and a second port. In FIG. 12, the first and second ports are labeled “In” and “Out”, respectively. However, the filter 1200 is bidirectional and either port may serve as the input or output of the filter. The two shunt resonators X2, X4 are connected from nodes between the series resonators to ground. All the shunt resonators and series resonators are XBARs.

The three series resonators X1, X3, X5 and the two shunt resonators X2, X4 of the filter 1200 maybe formed on a single plate 1230 of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a respective IDT (not shown), with at least the fingers of the IDT disposed over a cavity in the substrate. In this and similar contexts, the term “respective” means “relating things each to each”, which is to say with a one-to-one correspondence. In FIG. 12, the cavities are illustrated schematically as the dashed rectangles (such as the rectangle 1235). In this example, an IDT of each resonator is disposed over a respective cavity. In other filters, the IDTs of two or more resonators may be disposed over a common cavity. Resonators may also be cascaded into multiple IDTs which may be formed on multiple cavities.

Each of the resonators X1 to X5 has a resonance frequency and an anti-resonance frequency. In simplified terms, each resonator is effectively a short circuit at its resonance frequency and effectively an open circuit at its anti-resonance frequency. Each resonator X1 to X5 creates a “transmission zero”, where the transmission between the in and out ports of the filter is very low. Note that the transmission at a “transmission zero” is not actually zero due to energy leakage through parasitic components and other effects. The three series resonators X1, X3, X5 create transmission zeros at their respective anti-resonance frequencies (where each resonator is effectively an open circuit). The two shunt resonators X2, X4 create transmission zeros at their respective resonance frequencies (where each resonator is effectively a short circuit). In a typical band-pass filter using acoustic resonators, the anti-resonance frequencies of the series resonators are higher than an upper edge of the passband such that the series resonators create transmission zeros above the passband. The resonance frequencies of the shunt resonators are less than a lower edge of the passband such that the shunt resonators create transmission zeros below the passband.

Referring to the admittance versus frequency data of FIG. 7 and FIG. 8, it can be seen that the frequency offset between the resonance and anti-resonance frequencies of an XBAR with a 400 nm lithium niobate piezoelectric plate and a half-lambda dielectric layer is about 400 MHz. This frequency separation is not, of itself, sufficient for band-pass filters for telecommunications bands such as n79 (4400 MHz to 5000 MHz) and n77 (3300 MHZ to 4200 MHz). U.S. Pat. No. 10,491,192 describes the use of a dielectric layer deposited over shunt resonators to reduce the resonance frequencies of the shunt resonators relative to the resonance frequencies of the series resonators. U.S. Pat. No. 10,491,192 describes filters with very thin or no dielectric layer over series resonators and a dielectric layer thickness of about 0.25 times the piezoelectric plate thickness over shunt resonators.

A similar approach may be used to lower the resonance frequencies of shunt resonators relative to the resonance frequencies of the series resonators when the resonators are XBARs with half-lambda dielectric layers. In this case, the thickness tds of the dielectric layer over series resonators and the thickness tdp of the dielectric layer over shunt (parallel) resonators may be defined by

0.85λ_(0,d)≤2tds<2tdp≤1.15λ_(0,d).  (3)

Referring back to FIG. 8, the solid curve 820 is the admittance of an XBAR with a 375 nm SiO₂ layer over a 400 nm lithium niobate piezoelectric plate. The dashed curve 830 is the admittance of an XBAR with a 475 nm SiO₂ layer over a 400 nm lithium niobate piezoelectric plate. A filter, such as the filter 1200, could be fabricated using a 400 nm lithium niobate piezoelectric plate with 375 nm SiO₂ layer over series resonators and 475 nm SiO2 layer over shunt resonators. In this case, the frequency separation between the resonance frequency of the shunt resonators and the anti-resonance frequency of the series resonators will be about 800 MHz, which is sufficient for a band-pass filter for band n79. The frequency separation will scale proportionally with the thickness of the piezoelectric plate.

The ranges for the thickness of the SiO₂ layers over series and shunt resonators may be expressed in terms of the thickness ts of the lithium niobate piezoelectric plate as follows:

0.875ts≤tds<tdp≤1.25ts,  (4)

where tds is the thickness of the SiO₂ layer over series resonators and tdp is the thickness of the SiO₂ layer over shunt (parallel) resonators.

Description of Methods

FIG. 13 is a simplified flow chart showing a method 1300 for making an XBAR including partial Bragg reflectors or a filter incorporating such XBARs. The method 1300 starts at 1305 with a thin piezoelectric plate disposed on a sacrificial substrate 1302 and a device substrate 1304. The method 1300 ends at 1395 with a completed XBAR or filter. The flow chart of FIG. 13 includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown in FIG. 13.

The flow chart of FIG. 13 captures three variations of the method 1300 for making an XBAR which differ in when and how cavities are formed in the substrate. The cavities may be formed at steps 1310A, 1310B, or 1310C. Only one of these steps is performed in each of the three variations of the method 1300.

Thin plates of single-crystal piezoelectric materials bonded to a non-piezoelectric substrate are commercially available. At the time of this application, both lithium niobate and lithium tantalate plates are available bonded to various substrates including silicon, quartz, and fused silica. Thin plates of other piezoelectric materials may be available now or in the future. The thickness of the piezoelectric plate may be between 300 nm and 1000 nm. The piezoelectric plate may be, for example, Z-cut, rotated Z-cut, or rotated Y-cut lithium niobate or lithium tantalate. The piezoelectric plate may be some other material and/or some other cut. The substrate may be silicon. When the substrate is silicon, a layer of SiO2 may be disposed between the piezoelectric plate and the substrate. The substrate may be some other material that allows formation of deep cavities by etching or other processing.

In one variation of the method 1300, one or more cavities are formed in the substrate at 1310A, before the piezoelectric plate is bonded to the substrate at 1330. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using conventional photolithographic and etching techniques. For example, the cavities may be formed using deep reactive ion etching (DRIE). Typically, the cavities formed at 1310A will not penetrate through the substrate.

At 1330, the piezoelectric plate on the sacrificial substrate 1302 and the device substrate 1304 may be bonded. The piezoelectric plate on the sacrificial substrate 1302 and the device substrate 1304 may be bonded using a wafer bonding process such as direct bonding, surface-activated or plasma-activated bonding, electrostatic bonding, or some other bonding technique. The device substrate may be coated with a bonding layer, which may be SiO2 or some other material, prior to the wafer bonding process.

After the piezoelectric plate on the sacrificial substrate 1302 and the device substrate 1304 are bonded, the sacrificial substrate, and any intervening layers, are removed at 1340 to expose the surface of the piezoelectric plate (the surface that previously faced the sacrificial substrate). The sacrificial substrate may be removed, for example, by material-dependent wet or dry etching or some other process. The exposed surface of the piezoelectric plate may be polished or processed in some other manner at 1340 to prepare the surface and control the thickness of the piezoelectric plate.

Conductor patterns and dielectric layers defining one or of XBAR devices are formed at 1350. Typically, a filter device will have two or more conductor layers that are sequentially deposited and patterned. The conductor layers may include bonding pads, gold or solder bumps, or other means for making connection between the device and external circuitry. The conductor layers may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, molybdenum, tungsten, beryllium, gold, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between the conductor layer and the piezoelectric plate) and/or on top of the conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the conductor layers and the piezoelectric plate. The conductor layers may include bonding pads, gold or solder bumps, or other means for making connection between the device and external circuitry.

Conductor patterns may be formed at 1350 by depositing the conductor layers over the surface of the piezoelectric plate and removing excess metal by etching through patterned photoresist. Alternatively, the conductor patterns may be formed at 1350 using a lift-off process. Photoresist may be deposited over the piezoelectric plate and patterned to define the conductor pattern. The conductor layer may be deposited in sequence over the surface of the piezoelectric plate. The photoresist may then be removed, which removes the excess material, leaving the conductor pattern.

At 1360, a half-lambda dielectric layer may be formed on the front side of the piezoelectric layer. The half-lambda dielectric layer may be deposited over the conductor patterns or may be formed only between the fingers of the IDTs. In some filter devices, a first dielectric layer may be deposited over/between the fingers of all of the IDTs, and a second dielectric may be selectively formed over a portion of the IDTs, such as over only the IDTs of shunt resonators. The first dielectric layer will typically be thicker than the second dielectric layer. The first and second dielectric layers may be the same or different materials. Either the first or second dielectric layer may be deposited first.

In a second variation of the process 1300, one or more cavities are formed in the back side of the substrate at 1310B after all of the conductor patterns and dielectric layers are formed at 1350 and 1360. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using an anisotropic or orientation-dependent dry or wet etch to open holes through the back-side of the substrate to the piezoelectric plate.

In a third variation of the process 1300, one or more cavities in the form of recesses in the substrate may be formed at 1310C by etching the substrate using an etchant introduced through openings in the piezoelectric plate and half-lambda dielectric layer. A separate cavity may be formed for each resonator in a filter device. The one or more cavities formed at 1310C will not penetrate through the substrate.

In all variations of the process 1300, the filter device is completed at 1370. Actions that may occur at 1370 include depositing an encapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or a portion of the device and/or forming bonding pads or solder bumps or other means for making connection between the device and external circuitry if these steps were not performed at 1350. Other actions at 1370 may include excising individual devices from a wafer containing multiple devices, other packaging steps, and testing. Another action that may occur at 1370 is to tune the resonant frequencies of the resonators within the device by adding or removing metal or dielectric material from the front side of the device. After the filter device is completed, the process ends at 1395.

A variation of the process 1300 starts with a single-crystal piezoelectric wafer at 1302 instead of a thin piezoelectric plate on a sacrificial substrate of a different material. Ions are implanted to a controlled depth beneath a surface of the piezoelectric wafer (not shown in FIG. 13). The portion of the wafer from the surface to the depth of the ion implantation is (or will become) the thin piezoelectric plate and the balance of the wafer is the sacrificial substrate. The piezoelectric wafer and device substrate are bonded at 1330 as previously described. At 1340, the piezoelectric wafer may be split at the plane of the implanted ions (for example, using thermal shock), leaving a thin plate of piezoelectric material exposed and bonded to the acoustic Bragg reflector. The thickness of the thin plate piezoelectric material is determined in part by the energy (and thus depth) of the implanted ions. The process of ion implantation and subsequent separation of a thin plate is commonly referred to as “ion slicing”. After the piezoelectric wafer is split, the exposed surface of the piezoelectric plate may be planarized, and its thickness reduced, using, for example chemo-mechanical polishing.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items. 

1. An acoustic resonator device comprising: a substrate having a surface; a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate; an interdigital transducer (IDT) formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode in the diaphragm; and a half-lambda dielectric layer formed on one of the front surface and the back surface of the piezoelectric plate.
 2. The acoustic resonator device of claim 1, wherein a thickness is of the piezoelectric plate and a thickness td of the dielectric layer are defined as follows: 2ts=λ_(0,s), and 0.85λ_(0,d)≤2td≤1.15λ_(0,d), where λ_(0,s) is a wavelength of a fundamental shear bulk acoustic wave resonance in the piezoelectric plate, and λ_(0,d) is a wavelength of the fundamental shear bulk acoustic wave resonance in the dielectric layer.
 3. The acoustic resonator device of claim 1, wherein the dielectric layer is one or more of SiO₂, Si₃N₄, Al₂O₃, and AlN.
 4. The acoustic resonator device of claim 1, wherein the piezoelectric plate is lithium niobate, the dielectric layer is SiO₂, and a thickness ts of the piezoelectric plate and a thickness td of the dielectric layer are defined by the relationship: 0.875ts≤td≤1.25ts
 5. The acoustic resonator device of claim 4, wherein a temperature coefficient of frequency of the acoustic resonator device is between −32 ppm/C° and −42 ppm/C° at a resonance frequency and between −20 ppm/C° and −36 ppm/C° at an anti-resonance frequency.
 6. A filter device, comprising: a substrate; a piezoelectric plate having parallel front and back surfaces and a thickness ts, the back surface attached to the substrate; a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators including a shunt resonator and a series resonator, interleaved fingers of each of the plurality of IDTs disposed on respective portions of the piezoelectric plate suspended over one or more cavities formed in the substrate; a first dielectric layer having a thickness tds deposited between the fingers of the series resonator; and a second dielectric layer having a thickness tdp deposited between the fingers of the shunt resonator, wherein ts, tds, and tdp are related by the equations: 2ts=λ_(0,s), and 0.854λ_(0,d)≤2tds<2tdp≤1.15λ_(0,d), where λ_(0,s) is a wavelength of a fundamental shear bulk acoustic wave resonance in the piezoelectric plate, and ×_(0,d) is a wavelength of the fundamental shear bulk acoustic wave resonance in the dielectric layer.
 7. A filter device, comprising: a substrate; a lithium niobate piezoelectric plate having parallel front and back surfaces and a thickness ts, the back surface attached to the substrate; a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of resonators including a shunt resonator and a series resonator, interleaved fingers of each of the plurality of IDTs disposed on respective portions of the piezoelectric plate suspended over one or more cavities formed in the substrate; a first SiO₂ layer having a thickness tds deposited between the fingers of the series resonator; and a second SiO₂ layer having a thickness tdp deposited between the fingers of the shunt resonator, wherein ts, tds, and tdp are related by the equation: 0.875ts≤tds<tdp≤1.25ts.
 8. The filter device of claim 7, wherein a temperature coefficient of frequency of each of the plurality of resonators is between −20 ppm/C° and −42 ppm/C° at the resonance frequencies and the anti-resonance frequencies of all of the plurality of resonators.
 9. A method of fabricating an acoustic resonator device on a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to a substrate, the method comprising: forming a cavity in the substrate such that a portion of the single-crystal piezoelectric plate forms a diaphragm spanning the cavity; forming an interdigital transducer (IDT) on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm; and forming a half-lambda dielectric layer on one of the front surface and the back surface of the piezoelectric plate.
 10. The method of claim 9, wherein a thickness is of the piezoelectric plate and a thickness td of the dielectric layer are defined as follows: 2ts=λ_(0,s), and 0.854λ_(0,d)≤2td≤1.15λ_(0,d), where λ_(0,s) is a wavelength of a fundamental shear bulk acoustic wave resonance in the piezoelectric plate, and λ_(0,d) is a wavelength of the fundamental shear bulk acoustic wave resonance in the dielectric layer.
 11. The method of claim 9, wherein forming a half-lambda dielectric layer further comprises: depositing one or more of SiO₂, Si₃N₄, Al₂O₃, and AlN.
 12. The method of claim 9, wherein the piezoelectric plate is lithium niobate, and forming a half-lambda dielectric layer comprises depositing SiO₂ to a thickness td, where td is greater or equal to 0.875ts and less than or equal to 1.25ts, where ts is a thickness of the piezoelectric plate.
 13. The acoustic resonator device of claim 6, wherein the first dielectric layer and the second dielectric layer are one or more of SiO₂, Si₃N₄, Al₂O₃, and AlN. 